The present invention relates to a storage type electric water heater.
Electric water heaters are classified into two types, i.e., the instantaneous type and storage type. In the instantaneous type, an electric heater capable of providing great heating power is used, so that water is instantaneously heated to a desired temperature. On the other hand, in the storage type water heater, water heated to a desired temperature is beforehand stored in the water tank and discharged when required. The instantaneous type water heater requires an electric heater having great heating power, e.g., 5-20 kw., so that it can supply a sufficient amount of hot water. The storage type water heater is therefore chiefly used in ordinary houses.
The storage type electric water heater includes a water tank, the outer peripheral surface of which is covered with a heat insulator. The lower part of the water tank is connected to a cold water supply pipe, and the upper part of the same is connected to faucets through a hot water supply pipe. A sheathed electric heater is arranged at the lower part of the water tank. The electric heater is supplied with an electric current during a period in which hot water is seldom used, e.g., midnight hours, so that water in the water tank is in its entirety heated to, e.g., 80.degree. C. overnight. There are two ways for supplying an electric current to the electric heater: first, the supply of electric current to the heater is started at any necessary time and stopped when the temperature of water in the tank reaches a desired level; and secondly, the supply of electric current to the heater is started in midnight hours in which the user is charged less for the use of electricity, and stopped when the temperature of the water reaches a desired level. The latter (i.e., midnight power supply) is widely used for the purpose of economy. The water heater of this type includes a timer switch which is operated at a preset time of a day for control of the power supply to the electric heater. The timer switch is kept under control of the electric power company, so that the user may not manipulate the timer switch.
The storage type electric water heater, however, has the following problems. In the water heater of the first-mentioned power supply type, for example, when an electric current is supplied to the heater, the temperature of water in the water tank rises slowly because of natural convection. In this case, an increase in temperature of the water with respect to time depends upon the heating power of the electric heater and the capacity of the water tank. However, as stated above, it is not desirable to use an electric heater having great heating power in ordinary household appliances. On the other hand, in order to make full use of the storage type water heater, the water tank should desirably have a capacity of at least hundreds of liters. For this reason, the temperature of water in the water tank does not increase rapidly. When the temperature of the water reaches a desired level, e.g., 80.degree. C., the supply of electric current to the heater is stopped. Obviously, the electric water heater of the first-mentioned power supply type requires a longer time for increasing the temperature of the water to a level suitable for use (e.g., 80.degree. C.), thus failing to meet the requirement of high responsiveness. Further, as will be understood from the above, the water in the tank is in its entirety heated to the preset level (i.e., 80.degree. C.), regardless of the amount of water actually required. Therefore, if the necessary amount of water is 100 liters, and the water tank has a capacity of 300 liters, electric power is consumed wastefully by an amount required to increase the temperature of 200-liter water to 80.degree. C. This is the case with the electric water heater of the midnight power supply type. The water heater of the midnight power supply type employs a water tank having a great capacity, since the power supply period is fixed, and it is necessary to prevent hot water from running short during the daytime. Therefore, greater space is required for the water tank, and electric power is often consumed wastefully. Thus, the conventional electric water heater of the midnight power supply type can supply hot water of 80.degree. C. at any time in the daytime without delay, since water is heated and stored overnight. However, when the hot water is used up, the heater becomes useless.
Thus, with the conventional storage type electric water heater, it is not possible to quickly store a desired amount of hot water of, e.g., 80.degree. C., at a desired time. The conventional water heater has poor responsiveness and usability, and is likely to consume electric power wastefully.